Electrically controlled follow-up system



WAYNE B. MOORE ATTORNEY BY w W July 7, 1953 w. B. MOORE ELECTRICALLYCONTROLLED FOLLOW-UP SYSTEM Filed Feb. 26; 1948 4 Sheets-Sheet 2INVENTOR. WAYNE B. MOORE ATTORNEY July 7, 1953 w. B. MOOE ELECTRICAL-LYCONTROLLED FOLLOW-UP SYSTEM Filed Feb. 26. 1948 1 III Ill will r!!! July7, 1953 w. B. MOORE EL..ECTRICALLY CONTROLLED FOLLOW-UP SYSTEM 4Sheets-Sheet 5 Filed Feb. 26. 1948 "IIIIIIIIIIIIIIIIIIIII/ INVENTOR. 7B. MOORE WAYNE BY 1%. W

ATTORNEY Patented July 7, 1953 ELECTRICALLY CONTROLLED FOLLOW-UP SYSTEMWayne B. Moore, San Diego, Calif.

Application February 26, 1948, Serial No. 11,232

Claims. (01. 121-41) (Granted under Title 35, U. S. Code (1952),

see. 266) This invention relates to a follow-up system, and moreparticularly to a system by means of which a mechanical member such as apen may be made to follow rapidly and accurately the variations in anelectric signal. This invention has particularly advantageousapplication to the plotting on polar coordinate paper of the radiationpattern of a radio antenna.

It is an object of this invention to provide an improved system formoving a relatively heavy mechanical member accurately in accordancewith the strength of an electric signal, with minimum time delay betweenthe change in strength of the signal and the re-positioning of themember.

More particularly, it is' an object of this invention to provideapparatus for accurately and substantially instantaneously positioning agraphing pen, movable over polar coordinate paper, in accordance with acharacteristic such as magnitude of an electric signal, e. g. a signalradiated by an antenna.

It is another object of this invention to provide a follow-up systememploying compressed air to instantly and uniquely position a mechanicalmember in accordance with an electric signal.

It is a further object of this invention to provide improved valve meansfor directing a fluid selectively into either end of a cylinder whichcontains a driving piston, and to simultaneously exhaust, or vent toatmosphere, the other end of the cylinder.

Specifically, it is an object of this invention to provide such a valveas may be actuated in response to a characteristic of an electricsignal.

It is a further object of this invention to provide electric circuitmeans for energizing a pair of flux generating coils, which in turnactuate a valve to direct fluid selectively into either end of apiston-driving cylinder, the energization of the respective coils beingdetermined by the magnitude of an input electric signal.

It is an additional object of this invention to provide a valveactuatable selectively between two positions in response to theenergization of a pair of flux generating coils, which valve will meterthe flow of fluid through the valve in proportion to the arithmeticdifference in energization of the coils.

In accordance with these and other objects which will become manifest inthe following description, the follow-up system constituting the instantinvention comprises a mechanical member connected to be positioned by apiston reciprocable within a cylinder. Specifically, the mechanicalmember consists of an inking pen mounted on the end of an arm whichextends radially over a turn-table bearing a piece of polar graphingpaper. The outer end of the arm is suitably mounted for reciprocationradially of the turn-table, and is connected by suitable push rods tothe piston.

The cylinder is provided with fluid conduits communicating with itsrespective ends, the other end of each conduit communicating with avalve which is operable to supply fluid under pressure to either end ofthe cylinder, while simultaneously venting fluid from the opposite end.The valve may also assume a position wherein fluid flows to neither endof the cylinder, whereby the pen remains stationary.

The valve comprises essentially a valve body having an inlet conduit, bywhich fluid under pressure may be supplied to the valve and a pair ofoutlet conduits, by which the incoming fluid may be directed selectivelyto either end of the cylinder. Ferromagnetic valve means are mounted formovement back and forth between two positions to apply fluid selectivelyto either outlet conduit. This valve means is controlled in accordancewth the energization of a pair of flux generating or inducing coils.These coils are energized from an electric circuit capable of causing anelectric signal to control the balance of energization between twocoils, so that the valve will be actuated in such a manner as toposition the piston at a unique point, depending on the electric signal.

To effectuate such follow-up control, the electric circuit includes acompensating impedance which is positioned in a portion of the circuitthat is energizable in response to the incoming electric signal, andwhich is variable mechanically in accordance with the position of thepiston, to which it is connected by suitable mechanical linkage. Thiscompensating impedance most conveniently assumes the form of apotentiometer, positioned in a portion of the circuit that is energizedin response to the electric signal, and having its movable arm or sliderconnected to be positioned uniquely in response to the position of theinking pen, and hence of the piston.

The preferred actuating fluid is compressed air, and the descriptionhereinafter will illustrate the instant invention using that medium,although it is to be understood that many fluids are now known which aresuitable for this purpose.

Several modifications of the instant invention are illustrated in thedrawing, wherein:

Fig. 1 is an elevational view showing the marl:- ing pen and rotatabledisk assembly forming a portion of the instant follow-up system;

Fig. 2 illustrates partly schematically one form of valve forselectively driving the pen-connected piston back and forth;

Fig. 3 is a wiring diagram of a circuit suitable for energizing theactuating coils of the valve of Fig. 2;

Fig. 4 is a view similar to that of Fig. 2, show-- ing somewhatschematically an alternative type of valve which may be used to drivethe pen illustrated in Fig. 1;

Fig. 5 is an actual cross-section of a valve which functions lilze thevalve shown schematically in Fig. 4;

Fig. 6 shows a circuit for energizing the valve of Figs. 4 5. Thecircuit of Fig. 6 may also be used to energize the type of valve shownin Fig. 2, just as the Fig. 3 circuit may be used to energize a valve ofthe Fig. 4 5 type;

Fig. 7 shows still another alternative form of valve, which may besusbtituted for those of Figs. 2, 4, or 5, to control the pen-drivingmechanism of Fig. 1;

Fig. 8 illustrates a circuit suitable for energizing the control magnetsof the valve shown in Fig. 7.

Referring to Fig. 1, ll designates a vertically mounted marking pen, thepoint of which rides on the face of a turn-table l2 rotated on thevertical shaft it, which is driven through gears l4 and i5 by a suitablefollow-up or servomotor l5. In use, the motor I6 is generally driven atconstant speed, although this is not in the least essential to operationof the device.

Pen 1 l is secured at the end of an arm El, perforated at is forlightness, the other end of which is secured to a standard ii? forradial movement with respect to this disk 523. Standard I9 is mounted ona bracket El, near the top of which are pivoted a pair of wheels 22,which ride on a high rail 23 mounted by brackets 24 radially withrespect to the disk i2. To the bottom edge of bracket 2| is secured apush rod 25 extending diametrically underneath the turn-table l2 andterminating in a piston 26 (Fig. 2), reciprocable within a cylinder 2?.By proper application of driving air to the respective ends of cylinder21,

the push rod 25 may be reciprocated.

From the description thus far, it will be manifest that the radialposition of the point of pen 1 I above the turn-table i2 is determineddirectly by the position of the piston in the cylinder 27. Admission ofdriving air to the cylinder 2'! for determining the position of thepiston 23 is controlled by a valve 28, mounted for convenience atop thecylinder 2? and beneath the turn-table 12. Several suitable forms forthe valve 28 are illustrated in the succeeding figures; the first to bediscussed is that shown in Fig. 2, wherein 28 designates generally oneform of valve for use as the valve 23 in Fig. l.

The valve 28' comprises a body 3| to which air may be admitted throughan inlet conduit 32. A

pair of outlet conduits and 34 lead from the. valve 23' to therespective ends 35 and 38 of the cylinder 21. Ferromagnetic valve meansare pro- .vided for selectively diverting incoming air from greater thanthat on ball 38.

tor 45.

4 cable in a pair of somewhat elongate chambers 4i and 42 formed in thebody 3|.

Movement of the spheres or balls 31 and 38 is effected by a pair ofcoils 43 and 44 wound around respective U-shaped core pieces 45 and 46,positioned adjacent the chambers 4| and 42 so as to create magnetic fluxfields generally longitudinally of the chambers. The inlet conduit 32 isbifurcated into two branches 41 and 48, which pass conveniently throughthe pole pieces 45 and 4G, and terminate at one end of the respectivechambers 4! and 42 in inlet ports 5! and 52. respectively. Exhaust ports53 and 54, which vent to atmosphere, are provided respectively in theother ends of chambers 4| and 42. It will be noted that the coils 43 and44 are so disposed that when either generates flux the respective balls31 and 38 are pulled against the respective inlet ports 5| and 52 toblock admission of air from inlet conduit 32.

When air pressure is applied through conduit 32, and the coils 43 and 44are not energized, the balls 3i and 3B are driven upward and heldagainst the exhaust ports so that no air can flow. If the coils areequally energized the balls are attracted downwardly with equal force,thereby maintaining the balance in the system so that piston 26 does notmove and air flow is still blocked. Should the energization of the coilsbe unbalanced, however, the two balls do not assume balanced positionsand air is admitted to one side of the cylinder 21, while beingsimultanecusly exhausted from the other. For example, assume that themagnetic pull on ball 31 is The fact that air pressure on each ball isthe same results in ball 3'1 assuming a position lower than that of ball38. Air therefore flows from inlet 52 into chamber outlet conduit 34,and cylinder 21, where it impels piston 26 to the left. Simultaneouslyair is vented from the left hand end of cylinder 21, through conduit 33,chamber 4! and vent port 53. Converse operation occurs when the pull onball 37 is less than on ball 38.

If now means are provided for energizing coils 43 and 44 conversely, ordifferentially, so that current in one is made to increase as current inthe other is made to decrease, a control may be established which willopen one side of the valve simultaneously with closing of the otherside, or when currents are equal, will close both sides of the valve,thereby to control movement of piston 26 in accordance with energizationof the coils 43 and 44. Such a means is shown in the circuit of Fig. 3,wherein represents a terminal connected to the positive side of a highvoltage supply. Current from this supp-1y flows through a relativelyhigh resistance 61 and thence in parallel through two branches, thefirst of which consists of the valve coil 44 in series with a resis- Theother branch includes the coil 43, the plate cathode connections of agas filled tube 63, and a time constant circuit in the form of aparallel RC circuit 64-65. For purposes of example, it will be assumedthat the pen H is to be positioned in accordance with the field strengthof a rotatable antenna. A carrier wave modulated by a constant frequencyaudio signal is radiated by the antenna, which is rotated at somedistance from a receiver. The follow-up or servomotor I6 is driven inaccordance with the azimuth position of the antenna, which in turntherefore controls the angular position of the turn-table l2.

The received signal is demodulated, and the derived audio signal, thestrength of which is proportional to that of the carrier, is applied tothe input terminals 66 in Fig. 3. This signal is half wave rectified ina rectifier 61, and applied between the grid 68 of the tube 63, and thslider 69 of a potentiometer ll, connected between ground I8 and B plus.Rectification of the signal takes place in the rectifier 61 and theresistor 12 only on the negative swing, 'so that the potential on grid68 is essentially the positive bias at the slider 69 less theunidirectional voltage produced by the half Wave rectification of thesignal applied to the terminals 68.

In order that the follow-up system may function properly, it isessential that movement of the piston 26 operate in some fashion torestore the balance between the currents in coils 43 and 44. In theinstant invention this is done by moving the slider 53 along thepotentiometer H in accordance with the position of the piston 26. Asshown in Figs. 1 and 2, rack teeth 13 are formed on the rod 25, andengage a pinion 14 which is connected to the slider 69 of thepotentiometer H.

Tube 83 is gas filled and will continuously conduct after ionizationuntil the plate voltage drops to a low value. In order for grid 68 tomaintain constant control over the passage of current thrugh tube 63,the plate voltage must periodically drop to a value which will cause thetube to cease conducting. Since direct current is preferred for theproper operation of the solenoid which includes winding 44, a directcurrent plate supply is employed. It has been found that the negativeresistance characteristics of a gas tube may be utilized in anarrangement which includes resistor 64 and capacitor 65 connected inparallel in the cathode circuit and winding 43 with its distributedcapacitance 62 in the plate circuit to build up oscillations which areof sufficient ma nitude to cause the tube to cea e conducting when thecontrol grid becomes negative. The time constant of the cathode circuitis long with respect -to that of the plate circuit 50 that grid controlof the plate circuit of tube 83 is maintained over a range of platecurrent between zero and maximum. It will be understood that there arenumerous other suitable known methods for providing a pulsating directcurrent plate supply which will enable the grid in a gas tube tomaintain control of current flow through the tube.

An increase in signal voltage on terminal 66 increases the voltageacross the resistor 12, which, being opposed to the bias at slider 69-,and negative with respect to grid 68, lowers the net gridto-cathodevoltage on the tube 53. This decreases the average current through thecoil 43. The value of resistor 61 is sufficiently high to cause asubstantially constant total current to flow from the B supply terminal88, so that a reciprocal increase in current take place in the coil 44.The

consequent unbalance in currents in the two coils results in the ball 38being pulled downward to close the port 52, while the ball 31 is pushedupward by air pressure as a result of the decreased pull on that ball.The resulting air flow causes the piston 26 to move to the right in Fig.2, thereby moving the pen ll outwardly of the turntable l2.

The rightward motion of rod 25 moves the slider 69to the left in Fig. 3through the rack 13 and pinion 14. This movement continues until thepiston 26 has moved the slider 69 far enough to the left in Fig. 3 toraise the potential on grid 68 back to the equilibrium value, where thecurrent in coil43 equals that in coil 44. The posi-' tion of piston 26and hence of pen II is thus determined by the magnitude of the signalapplied to the terminals 66.

Operation The complete operation of the apparatus shown in Figs. 1, 2,and 3 will now be reviewed. A signal of given amplitude applied toterminals 66 (Fig. 3) is just correct to pass an average current throughtube 63, equal to that flowing through coil 44. The magnetic pulls onthe balls 31 and 38 are thus equal, and since the air pressure at 5| isthe same as at 52, the balls 31 and 38 assume identical position withinthe chambers 4| and 42, respectively. The equalized pressure on eachside of the piston 26 holds the pen H at a given radius on the rotatingturn-table [2, corresponding to the amplitude of the signal at 66.

The potential on grid 68 (Fig. 3) i equal to the algebraic sum of apositive component embodied in the voltage drop between slider 69 andground 10, and a negative component embodied in the half wave rectifiedcurrent flowing through resistor 12. Assume that the signal on terminals36 decreases. This decreases the negative component of the potential ongrid 68, thereby increasing the average current through tube 63 and thecoil 43, and decreasing the current through coil 44. The result is anunbalance between the balls 31 and 38; the former being pulled down toshut off conduit 33 from inlet port 5| and connect it to exhaust port53; and the latter being pushed up by air flow to cut off exhaust port54 and admit air to the right hand side of piston 26. The resultingleftward movement of piston 28 (Figs. 1 and 2) moves pen II to the left,indicating, as it should, a decrease in signal strength. Through rack 13and pinion 14, the slider 59 is moved to the right (Fig. 3), therebydecreasing the positive component of the potential on grid 68, until theaverage current through tube 63 becomes equal to that through the coil44. At this point the magnetic pull on the respective balls 3'! and 38is equalized, and the valve 28 again becomes balanced, with the balls 31and 38 occupying identical positions in their chambers 41 and 42,respectively. The piston 26 has moved leftward to a positioncorresponding to the new, smaller signal value, where it remains untilanother variation in signal strength takes place.

First modification A first modified form of valve and circuit which maybe substituted interchangeably for the above described valve and circuitis shown in Figs. 4, 5, and 6. Referring to Fig. 4, the valve 28" isshown having a valve body Bl of reduced upper portion 82. In oppositefaces of the main portion of the body 8| are formed chambers 83 and 84,in which reciprocate ferromagnetic balls 85 and 86, respectively. As inthe case of the Fig. 2 valve, the inlet conduit 32 is bifurcated intotwo branches, 47 and 48, communicating respectively with the ends of thechambers 83 and 84. By means of a connect: ing conduit 81, the chamber84 is connected at one side thereof to outlet, or cylinder connecting,conduit 33. Chamber 83 is similarly connected to outlet conduit 34 bymeans of a connecting conduit 88, which crosses the conduit 81. In thereduced body portion 82 is formed a through bore constituting an exhaustchamber 9| in which a ball 92 reciprocates. The ends of the chamber 9!communicate respectively with conduits 33 and 34; while at the'slde ofthe chamber BI there is provided an exhaust port 03.

Onopposite sides of the valve body 01 are mounted W-shaped pole pieces94 and 05, the middle legs of which mount the coils 43 and 44,respectively. The outer legs of the pole pieces 94 and 95 terminateadjacent the chamber and the chambers 83 and 64, respectively, so thatcurrent through one of the coils, 43 for example, pulls the ball 85 tothe left in chamber 83, shutting off the inlet branch 41, and also pullsthe ball- 92 to the left in chamber 3| to close that chamber from theconduit 33 and simultaneously connect the conduit 34 with the outletport 93 through the chamber QI. Under thi condition, air from inletconduit 32 fiows into branch 48, chamber 84, connecting conduit 81,outlet conduit 33 and thence into the left side of cylinder 21, where itpushes piston 26 rightward. The exhausting air in front of piston 26flows through conduit 34 and out exhaust port 63.

Converse operation occurs when the energization of coil 44 exceeds thatof coil 43, just as in the case of the valve of Fig. 2. Likewise, inmanner similar to that of Fig. 2, an intermediate, balanced position isassumed by the balls 85, 86, and 92 whenever the currents in coils 43and 44 are equalized. The actual valve body ill of Fig. 4 preferablyassumes the configuration shown in Fig. 5, wherein the primed numeralscorrespond to the same numerals unprimed in 1-. The relation betweenthese two types of valve bodies (Figs. 4 and is so close that furtherdescription will be omitted. The actual reciprocating travel of theballs 85, 86', and 32 is very short, in the order of a few thousandthsof an inch.

A suitable circuit for actuating the valve of Figs. 4-5 is shown in Fig.6, wherein the controlling signal is applied to the input terminal 66amplified twice in the dual tube I0! and rectified in rectifier I02. Therectified signal from I02 is applied between the plate of a spill-overdiode I03 and the slider 69 of the potentiometer 1I. tial at the diodeplate I04 has two components. There is a positive component resultingfrom the voltage drop created by the B+supply I05 between the slider 69and ground I06. There is another component, in this case also positive,re-

sulting-from the half wave rectified current flowing through theresistor I01. The cathode E08 of diode I03 is backed off to apredetermined positive voltage, by being returned through a resistor I09to the slider III of another potentiometer II2 also connected between Bplus I05 and ground I06. Thus, a degree of conduction is maintained inthe diode I03 which is variable in accordance with the magnitude of therectified voltage appearing across resistor I01.

A constant frequency audio oscillator II3 has its output connectedthrough a capacitor II4 to the cathode I08 of the diode I03. Theamplitude of the audio signal which is enabled to pass through the diodeI03 to allect the grid I I5 of an amplifier H6 is thus determineddirectly by the backing off current through the diode I03 resulting fromthe two biases at 69 and I01, respectively. The latter, rectified andsmoothed out by a filter II1, is substantially a D. C.

That portion of the oscillator signal which passes through rectifier I03is amplified in amplifier H6, and is divided and fed into two half waverectifiers I22 and I26. respectively. The rectifier I22 passes positivehalf cycles which As in the case of the Fig. 3 circuit the poten- M arefiltered at I23 and fed to the grid I24 of a tube I25, in the platecircuit of which is connected the coil 44. The negative half cycles passthrough the rectifier I26, are filtered at I21, and are applied as anegative bias to the grid I of a tube I29 in the plate circuit of whichis connected the coil 43. -The grid I24 of tube I25 is given anadditional, negative bias by virtue of the current in cathode resistorI3I, so that for a predetermined amount of audio signal spilloverthrough rectifier I03, the net grid voltages on I24 and I23,respectively, are equal, thereby causing equal currents in the coils 43and 44.

Operation of first modification The operation of the valve of Fig. .4 ascontrolled by the circuit of Fig. 6 will now be described, it beingunderstood that either of these portions of the system may besubstituted individually for the corresponding portions shown in Figs. 2and 3 respectively.

With a given signal on the input terminal 66, currents in coils 44 and43 are equal. thereby maintaining balls 86 and 85 in balanced, identicalpositions, sothat air pressure is equalized on the piston 26. Assume thesignal on terminal 66 increases. This increases the rectified component(across resistor I01) of the voltage on the plate I04 of the diode I03.Increased conductivity of the diode I03 allows more of the signal ofoscillator II 3 to be applied to the grid II5. This increases the outputof the amplifier II6, the positive half cycles of which pass throughrectifier I22 to increase the grid-cathode bias on grid I24.Simultaneously the negative half cycles of the output from I I6 passthrough rectifier I26, to decrease the grid-cathode bias on the gridI28. This causes an increase in current through coil 44 and a decreasethrough coil 43. The resulting unbalance in the energizing currents invalve 28 causes balls 85, 86, and 92 to move rightward from theirpositions of equilibrium; and allows air to flow from inlet conduit 32thru conduits 41, 68, and 34 into the right hand side of cylinder 21pushing piston 26. outward and causing pen I I (Fig. 1) to move to theright.

Through rack 13 and gear 14, slider 69 (Fig. 6) of potentiometer H iscaused to move downward, thereby decreasing the potentiometer componentof the voltage applied to the plate I04 of the diode I03. Motion of theslider 69 ceases when the net voltage (D. C.) on the plate I04 is justsufficient to allow the predetermined, correct amount of oscillatorsignal to spill through diode I03 to maintain equilibrium of currents incoils 43 and 44. This equilibrium restores the .balls in valve 26 to abalanced position, wherein air is neither admitted to, nor vented from,cylinder 21.

Second modification A second modified form of valve and circuit, whichmay also be substituted for the valves and circuits above described, isshown in Figs. 7 and 8. The valve 28 in Fig. '1 has an inlet conduit 32and outlet conduits 33 and 34, just as in the case of the previouslydescribed valves. The inlet conduit, however, is not bifurcated asbefore, but is led directly upward through a metering valve I4I, to bedescribed hereinafter, and terminates in a port I42. The conduits 33 and34 likewise terminate in ports I43 and I44 positioned on opposite sidesof the port I42. The terminating face of the three ports is concavelyarcuate, so that a valve member I45 may rock back and forth across thefaces of the ports, to selectively apply incoming air from port I42,either to port I43 or to port I44, while simultaneously venting theunselected port to atmosphere. This it does by virtue of a nether recessI46 which forms a junc ture conduit between the ports.

Member I45 is secured to the end of a ferromagnetic arm I41, pendantlypivoted at I48 and provided with oppositely disposed fiexure arms I49and II which bear against the pole pieces I52 and I53, respectively,around which the coils 43 and 44 are wound. The flexure members I49 andI5I are of equal strength, so that with balanced currents in coils 43and 44, they tend to maintain the arm I41 in mid-position. When,however, the coil currents become unbalanced, the arm I41 is pulled toone side or the other, depending on the algebraic difference in coilcurrents, thereby causing the piston 26 to move in the appropriatedirection.

The valve 28" in Fig. '1 includes a metering valve I M not found in thepreviously described embodiments. The valve I 4| is actuated by a coilI55, which is energized in inverse proportion to the arithmeticdifference between the currents in coils 43 and 44. Thus, when a largecurrent unbalance occurs, irrespective of the direction of theunbalance, the valve MI is opened wide to admit a large flow of airthrough the conduit of 32, which drives the piston 26 rapidly. As thedegree of unbalance decreases, the current in coll I55 increases,thereby gradually closing the valve MI and throttling the flow of airthrough the inlet conduit 32.

The valve I4I embodies certain features which make it uniquelyapplicable for use as a metering valve. These features are such that theclosing force on that valve is made non-linear in relation to the fluxproduced by coil I55, so that when the valve is wide open, a smallincrease in flux produces a marked throttling by the valve, but when thevalve is nearly closed, a very much larger flux is needed to furtherincrease the throttling effect.

These features of the valve I M will become readily manifest in thefollowing description, wherein I56 designates a valve chamber having aninlet port I51 fed by the conduit 32. The outlet port connects with thecontinued extension of the inlet conduit 32. A ball I58 rests againstthe port I51 and is impelled away from the port by the incoming airpressure. A lever I59 pivoted at I6I rides against the top of the ballI58, and is in turn pressed downward by a ferromagnetic lever orarmature I62'pivoted at I63, which is actuated by the pole pieces I64,magnetically energized from the coil I55. It Will be noted that one ofthe levers, in the illustrated case, lever I59, is curved convexly withrespect to the other, so that the fulcrum point of the lever I62 againstthe lever I59 shifts continuously toward the pivot I 6|, and away fromthe pivot I63, as the lever I62 is pulled downwardly against the leverI59. It will be manifest that when the magnetic pull is small, thefulcrum point between the two levers is substantially directly over theball I58 at the left hand end of lever I59. However, as the lever pivotsdownward, the fulcrum point shifts steadily to the right, therebydecreasing the mechanical advantage of the lever I62 against the leverI59. This is due to the double effect of the fulcrum point movingsimultaneously away from the pivot I63 and toward the pivot I6I. Eitherone of these effects would produce a similar non-linear resistance, butby connected the coil 43.

10 cumulating them the effect may be made more sharply effective.

A suitable circuit for actuating the coils 43, 44 and I55 of Fig. '1 isshown in Fig. 8, wherein an incoming signal applied to terminal 66 isamplified at I1I, rectified at I12, and applied as a D. C. in serieswith the compensating slider 69. The potentiometer H on which the slider69 rides is connected between B plus and ground to form the positivecomponent of bias while the signal voltage rectified in I12 forms thenegative component, just as in Fig. 3. This bias is applied to the gridof an amplifier I13, the output of which is filtered at I14 and appearsat point I15 as a uni-directional, of D. C. potential, varyingsubstantially proportionally with the magnitude of the A. C. signal at66.

A pair of tubes I16 and I11 are connected through a common cathoderesistor I18. The grid I19 of the tube Ill is maintained at asubstantially constant potential with respect to ground by beingreturned to slider I8I of a potentiometer I82. The grid I83 of the tubeI16 is energized in accordance with the varying D. C. voltage at pointI15. For a predetermined voltage at I15, the plate potentials of I16 andI11 will be equal. A rise in the voltage at I15 causes a correspondingdrop in plate potential at I84; and by virtue of the increased currentin resistor I18, produces an increase in bias on the cathode of tube I11which decreases the current therethrough and raises the potential at theplate I 85. The tubes I16 and I11 are thus connected in What might becalled a seesaw circuit, wherein one voltage drops as the other rises,and vice versa; and wherein the voltages are equal only for, apredetermined positive value of voltage at point I15.

The plate potential at I84 is applied to the grid I86 of a tube I81 inthe plate circuit of Whichis The plate potential at I is applied to thegrid I88 of a tube I89, in the plate circuit of which is connected thecoil 44.

The balancing operation of the Fig. 8 circuit is substantially the sameas that of the circuits hereinbefore described. For example, an increasein signal strength at terminal 65 produces an increased output from therectifier I12 which appears as more negative bias on the tube I13. Thisraises the potential at the plate of tube I13. The resulting increase inpotential at point I15 drops the voltage at I 84 and, by virtue of thecommon cathode resistor I18, raises the voltage at I85. This causes adecreased current through the tube I 81 and the coil 43, and anincreased current through the tube I89 and the coil 44. Valve member I45is pulled to the right, causing the piston 26 to move outwardly of thecylinder 21, thereby moving the pen II to the right, in Fig. 1. I

The coil I55 is energized in the following manner. The point I15 isconnected to the grid I9I of a tube I92, which has a common cathoderesistor I93, with a tube I94, the grid I of which is held at asubstantially constant potential with respect to ground by beingreturned to slider I8I. A pair of oppositely poled series connectedrectifiers I96 and I91 are bridged between the respective plates of thetubes I92 and I94; and shunting these rectifiers are series-connectedresistors I98'and I99, the juncture point 28I between the rectifiers I96and I91 being connected to that between resistors I98 and I99. Withequal voltages on the plates of tubes I92 and I94, there will be nocurrent in the resistors I98 and aecacec I99. Should the plates becomeunbalanced, a current flow will take place between them which will causethe point 2iiI to drop in potential, regardless of the polarity of theunbalance. This is because of the action of the rectifiers I96 and I91which in effect shorts out one or the other of the resistors I 98 orI99, depending on the polarity of the unbalance. For example, assumethat the plate potential of tube I54 rises while that of I92 fallscorrespondingly. Current thus flows through resistor I98, but isby-passed around resistor I99, by the rectifier I91. Point ZOI thusdrops in potential, in accordance with the arithmetic difference betweenthe plate potentials of tubes I92 and I94. An unbalance in the otherdirection would also cause a drop in potential at point 2I1I by reasonof the shunting action of rectifier I96 around resistor I98.

Point ZEII is connected to the grid 202 of a tube 203, in the platecircuit of which is the coil I55 of the valve I II. Thus, the arithmeticunbalance in the system produced by a change in the incoming signal atterminal 65 appears as a proportional decrease in current in the coilI55, which allows the air pressure to lift the ball I58 and permit agreater flow of air through the valve 28'.

Operation of second modification The operation of the valve of Fig. '7,as energized by the circuit of Fig. 8, will now be described. For agiven net bias on the tube H3, for a given voltage at point H5, currentbalance exists in tubes I76 and IT! and in tubes Hi2 and I94. Theresulting balance in currents in coils 43 and 44 holds arm It? atmid-position. wherein incoming air is blocked, being directed neither toconduit 33 nor to conduit 34. By virtue of the balanced condition, thevoltage at point 2%! is maximum positive being equal to voltage at theplates I92 and IM. The current in tube 283 is thus a maximum, so thatcoil I55 is given maximum energization and maximum pull is applied tothe armature lever I52. Ball I56 is thus held at its lowest or mostthrottling position.

Assume that the signal. at '55 decreases. This lessens the negative, orrectified, component of the bias applied to the grid of tube I13. Acorresponding increase in current flow occurs, which drops the platepotential of I13 causing a drop in potential at the point Iili. The dropat point I15 appears as an increased voltage at I84, and a. decreasedvoltage at i635, which causes increased current in the tube iii? anddecreased current in the tube I23. The resulting unbalance inmagnetizing flux on the arm Mi pulls the valve member I45 to the left,allowing air to enter the conduit 33 and exhaust from the conduit 34.This moves piston 26 inward (to the right in Fig. 7) and causes pen IIto move in toward the center of the turn-table I2 (Fig. l). Theresulting motion of slider 69 (downward in Fig. 8) decreases thepositive component of the bias on the grid of tube I13. Movement of thepiston 28 continues until the grid of H3 has been returned to its stablevalue, whereupon valve member I45 resumes its mid-position and thepiston 26 stops.

Simultaneously with the above rebalancing cf the system, the valve I iihas been operated in the following manner. Decrease of the voltage at Hraises the voltage on the plate of tube I82 and drops the voltage on theplate of tube I94. The resulting current flow through resistor I89 andrectifier I96 lowers the voltage at point 28 I, which is translated intoa current decrease in the tube M3 and the coil I5 The decreased pull onthe armature lever I62 allows the incoming air to push the ball 558farther away from the seat I51, thereby diminishing the throttlingaction of the valve I iI. As balance is restored, the potential at point575 returns steadily to its stable value, causing a steady increase inthe pull exerted by coil 55 on armature lever I 52, so that by the timefull equilibrium is established, the ball I58 has been pulled downalmost fully against the seat 357. The action of valve I4I thus tends toprevent overswinging of the piston 26, and consequent hunting within thesystem.

Several. embodiments of this invention have been described herein; itwill be understood that additional modifications may be made in thisinvention without departing from the spirit and scope thereof as setforth in the appended claims.

The invention herein described may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed l. Follow-up mechanism for positioning a mechanicalmember in accordance with an audio frequency modulated radio frequencysignal, comprising a track, a bracket having wheels riding on saidtraclr, a reciprocable rack secured'to said bracket, a pinion meshingwith said rack, a piston attached to said rack to reciprocate the sameon said track, a cylinder encompassing said piston, conduits foradmitting driving fluid to said cylinder to control the position of saidpiston, valve means in said conduits for controlling the admission offluid to said cylinder, and electric means. connected to control theoperation of said valve means in accordance with the signal to befollowed, said electric means including amplifying means eifective toamplify said signal, means'to demodulate said amplified signal, apotentiometer having two winding terminals and one slider terminal,means to connect said demodulated amplifled signal to a first windingterminal of said potentiometer, means to connect a counter voltage to asecond winding terminal of said poten tiometer, at least one balancingcircuit having an input terminal and a pair of output terminalseffective to deliver two output voltages having varying amplitudesrelative to each other depending on the extent and direction of thedeviationof the voltage applied to said input terminal from apredetermined balance voltage which is effective to produce equalvoltages said pair of output terminals, means for connecting the sliderterminal of said potentiometer to said balancing circuit inputterminals, means to amplify the outputs of said balancing circuits,solenoids for actuating said valve means in response to the amplifiedoutputs of said balancin circuits, said potentiometer having its slideradapted to be positioned by said pinion whereby follow-up control oversaid mechanical member is exerted to compensate for changes in thestrength of said. signal.

2. A follow-up system for positioning a mechanical member accordancewith an electric signal comprising a member to be positioned, a pair ofelectric solenoids constructed and arranged to control movement of saidmember in either of'two directions depending upon the algebraicdifference in energization applied to said solenoids, respectively, athird electric solenoid constructed and arranged to be energized tocontrol the speed. of movement of said member in both directions, afirst balancing circuit having an input terminal and a pair of outputterminals, effective to deliver to said output terminals equal voltagesfor a given input voltage, one of said output terminal voltages droppingand the other rising when the input voltage rises above said givenvoltage, and vice versa, a first amplifier connected to the inputterminal of said first balancing circuit effective to deliver thereto avoltage varying in response to an electric signal, a second amplifieradapted to receive the output from one terminal of said first balancingcircuit and deliver an energization current to one of said pair ofsolenoids, a third amplifier adapted to receive the output from theother terminal of said first balancing circuit and deliver anenergization current to the second of said pair of solenoids, a secondbalancing circuit having input terminal and a pair of output terminals,eiiective to deliver to said output terminals equal voltages for a giveninput voltage, one of said output terminal voltages dropping and theother rising when the input voltage rises above said given voltage, andvice versa, means to connect the input terminal of said first balancingcircuit to the input ter= minal of said second balancing circuit, animpedance shunting the output terminals of said second balancingcircuit, a pair of series connected, oppositely poled rectifiersshunting said impedance, the juncture between said rectifiers beingconnected to an intermediate point on said impedance, a fourth amplifierhaving an input lead connected to said intermediate point and adapted todeliver an energization current to said third solenoid.

3. Apparatus in accordance with claim 2 where in each said balancingcircuit comprises a pair of tubes having their cathodes connected to acommon impedance in their respective cathode ground circuits, wherebycurrent increase in one tube raises the cathode potential of the othertube to effect current decrease in said other tube, and vice versa, asource of plate supply for said tubes, means biasing the grid of saidother tube to a substantially constant potential with respect to ground,circuit means connecting the plates of said pair of tubes, respectively,to the output terminals of said balancing circuit, and circuit meansconnecting the grid of said one tube to the input terminal of thebalancing circuit.

4. A follow-up system for positioning a mechanical member in accordancewith an electric signal, comprising a movable member to be positioned, apair of electric solenoid windings energizable to control movement ofsaid member in dependence on the relative magnitudes of the energizingpower applied to the respective solenoid windings, and an electriccircuit for energizing said solenoid windings in accordance with anelectric signal, said circuit comprising an oscillator, a rectifier andpotentiometer paralleling said oscillator, an amplifier connected to theoutput of said oscillator, means for applying a unidirectional voltageproportional to said electric signal in series with said potentiometerto back off said rectifier and thereby to determine the amount ofoscillator output to said amplifier, the slider of said potentiometerbeing connected to be positioned in response to the position of saidmember, whereby movement of said member also varies the amount ofoscillator output applied to said amplifier, and means for energizingsaid electric solenoid windings in accordance with the output of saidamplifier.

5. A follow-up system for positioning a mechanical member in accordancewith an electric signal, comprising a movable member to be positioned, apair of electric solenoid windings energizablc to control movement ofsaid member in dependence on the relative magnitudes of the energizingpower applied to the respective solenoid windings, and an electriccircuit for energizing solenoid windings in accordance with an electricsignal, said circuit comprising an oscillator, a rectifier andpotentiometer paralleling said oscillator, an amplifier conn cted to theoutput of said oscillator, means for applying a unidirectional voltageproportional to said electric signal to said potentiometer to back oiisaid rectifier and thereby to determine the amount of oscillator outputapplied to said amplifier, the slider of said potentiometer beingconnected to be positioned in response to the position of said member,whereby movement of said member also varies the amount of oscillatoroutput applied to sai amplifier, a pair of tubes having their gridsconversely connected to the output of said amplifier whereby increasedamplifier output increases the conductivity of one tube while decreasingthat of the other, and vice versa, and circuit means connecting saidpair of electric solenoid windings, respectively, to the plate outputsof said pair of tubes.

WAYNE B. MOORE.

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